Example 1: The Car Class

Suppose you need to write a traffic simulation program that watches
cars going past an intersection. Each car has a speed, a maximum
speed, and a license plate that uniquely identifies it. In
traditional programming languages you'd have two floating point and
one string variable for each car. With a class you combine these
into one thing like this.

These variables (licensePlate, speed and
maxSpeed) are called the member variables,
instance variables, or fields of the class.

Fields tell you what a class is and what its properties are.

An object is a specific instance of a class with
particular values (possibly mutable) for the fields. While a class
is a general blueprint for objects, an instance is a particular
object.

Note the use of comments to specify the units. That's important.
A unit confusion between pounds and newtons led to the
loss of NASA's $94 million Mars Climate Orbiter. (Believe it or
not that's a cheap mission by NASA standards. If you're rich enough
that you don't have to worry about losing $94 million worth of
work, you don't have to put comments in your source code. Everybody
else has to use comments.)